Patent document 1 discloses a calculation to specify a lane marker based on data on reflection from a road surface using a lidar (i.e., laser radar) instrument mounted in a vehicle. Here, data on a plane parallel with a road surface are subjected to a frequency distribution in a vehicle's axle directional component. Data included in a certain segment where the frequency reaches the local maximum value is extracted as data on reflection from a lane marker. The extracted data is then subjected to a method of least square or Hough's transformation to thereby compute a parameter to specify the lane marker.                Patent document 1: JP-A-2005-222538        
Patent document 1 assumes that a lane marker has an approximately linear shape. When a road curves with a large amount of curvature, data on reflection from a lane marker cannot be accurately extracted, resulting in mis-recognition of a lane marker.